nix-instantiate: Add a --no-readonly-mode flag

This allows running nix-instantiate --eval-only without performing the
evaluation in readonly mode, letting features like import from
derivation and automatic substitution of builtins.storePath paths work.

Signed-off-by: Shea Levy <shea@shealevy.com>
Rename --no-readonly-mode --read-write-mode

Signed-off-by: Shea Levy <shea@shealevy.com>
nix-instantiate --eval-only --read-write-mode: Don't depend on ordering

Signed-off-by: Shea Levy <shea@shealevy.com>